The long-range objective is to understand enough about the factors that control mammalian cell growth to be able to apply this knowledge to the control of malignant growth. As a result of recent work, a broad picture is now available of the control of growth of mammalian cells. Application of this knowledge requires additional information and experimentation. Detailed information is needed of the specific factors that control the growth of the epithelial cells that are most important in the origin of tumors, in order to design rational procedures to restrict tumor growth. It is important to know whether these epithelial cells produce inhibitors of their own growth, such as the growth inhibitors that are produced by kidney epithelial cells. Isolation of the kidney determines whether the inhibitors can be used to restrict the growth of kidney tumors. These questions will be studied using methods of cell culture to investigate growth controls, using ultrafiltration, chromatography and other biochemical procedures in the isolation of the inhibitors, and using cell free systems, cell culture and work in intact animals to investigate the properties of the inhibitors.